The present invention broadly relates to a method of cleaning a spent fuel assembly which has been removed from service in a nuclear reactor. It particularly relates to a method of cleaning such a spent fuel assembly contaminated with a radioactive alkali metal and in which the fuel assembly comprises a spent fissile material selected from the group consisting of oxides and carbides of uranium and/or plutonium or a metallic uranium/plutonium alloy.
The typical breeder reactor utilizes an alkali metal as a coolant. In operation of the reactor a primary coolant is circulated via a pump through a core of fuel assemblies and then through a heat exchanger from which thermal energy is extracted by indirect heat exchange with a secondary coolant. The primary coolant picks up a certain amount of radioactivity in passing through the core. All components in contact with this primary coolant also become contaminated with the radioactive constituents. Thus, any component in contact with such coolant after removal from service must be cleaned, not only to remove the chemically highly reactive coolant but also the radioactive coolant.
All fuel handling of spent core assemblies must be done remotely in an inert gas filled fuel handling cell or facility such as a fuel storage building or housing, until the spent core assemblies are cleaned of sodium. Thereafter, they may be handled remotely in an air filled fuel handling storage facility or stored under water.
One method utilized for cleaning such components is to wash them with an organic solvent such as alcohol or wash them with water. The disadvantage of these methods is that they produce a large volume of liquid, low-level radioactive waste with its attendant storage and disposal problems.
Another technique utilized is to heat the contaminated parts to a sufficient temperature to volatilize the alkali metal and the radioactive constituents.
It also has been proposed to place such contaminated components in a chamber maintained under a vacuum to removal alkali metal vapors and radioactive constituents.
A fuel assembly removed from a breeder reactor presents a particularly unique problem. For example, during service the cladding of some of the individual fuel pins comprising the fuel assembly could have been cracked or ruptured such that some of the alkali metal will be within the cladding of the individual pins. In addition, the spent fuel assembly, after removal from service, will continue to generate some heat generally referred to as decay heat. Thus, if the fuel assembly was placed in a vacuum the individual pins could generate sufficient heat to rupture the cladding of those pins which were still intact with the release of more radioactive material. Clearly, the use of external heating would be equally inapplicable. Obviously there is a need for a procedure uniquely adapted to the cleaning of a spent fuel assembly which has been in contact with an alkali metal coolant.